This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Objective: To determine the impact of sequence polymorphisms of the 2009 H1N1 pandemic influenza virus on pathogenicity in a translational model. Influenza viruses attach to host cells using the hemagglutinin (HA) protein. Sequence changes in HA are known to affect the species and types of cells that viruses can infect and can also affect pathogenicity. During the 2009 pandemic, clinical investigators noted a potential association between H1N1 viruses bearing particular HA mutations and cases of lethal influenza. One frequently detected mutation encoded and aspartate-two-glycine substitution at HA amino acid 225 (D225G). This substitution had been observed in previous influenza virus isolates, but its role in causing severe disease has been difficult to determine in human patients. We therefore used our newly developed nonhuman primate model of influenza to determine the impact of the D225G mutation on viral tropism and disease severity. We infected macaques with pairs of viruses whose sequences differed at HA position 225 and assessed the severity of symptoms and extent of virus replication. We euthanized animals seven or 14 days after inoculation to assess influenza virus loads in lung tissue and to perform full microscopic and microscopic pathological analyses. We determined that in at least some genetic backgrounds the D225G substitution can result in severe disease in macaques. This severe pathology is associated with viral infection of type II pneumocytes, which are important in healing of lung damage. These findings, together with in vitro data and results from other animal models, suggest that the D225G mutation may be a virulence determinant in humans. A manuscript describing these results is under review. This research used CPI, Immunology &Virology Services and Genetics Services. PUBLICATIONS: None.